1. Field of the Invention
The present invention relates to network printing systems, and in particular, to a method and apparatus for calibrating digital halftoning algorithms with multiple personalities.
2. Description of the Related Art
Network printing systems generally comprise an assemblage of different printers, client computers, servers, and other components connected over a network. A print job is assembled at a client computer and transmitted over the network to a server linked to a variety of printers. Although these printers have historically been used to reproduce text, they are increasingly being used to reproduce graphic and image files, which have significant grayscale content.
Digital printers with grayscalc reproduction capabilities use half tone screens with or without dot density control (contone) to reproduce the grayscales. Due to differences printer personalities such as dot gain characteristics, types of screens, and contone designs, grayscale profiles can vary widely from printer to printer. Consequently, when graphic and image files generated for a particular grayscale profile are sent to a printer with a different profile, the appearance of the printed output may be substantially different. Such variations are more pronounced among bi-level printers, because dots are not as easily controlled as they are with contone printers. These variations often result in visually unacceptable results, because proper rendering of grayscale information is critical to the reproduction of graphical and image files.
One way to account for these variations is to pass grayscale command inputs through a transfer function to increase or decrease each grayscale command before beginning the halftone process. This transformation calibrates the printers by modifying the uncorrected grayscale inputs to values that, when provided to the printer, will result in the desired printed grayscale output. The transfer function is typically defined from the differences between commanded grayscale images and densitometer measurements. The transfer functions are implemented in the computer or server submitting the print job to the printer, or the printer itself.
In addition to the base transfer function required to account for the nominal dot gain and other printer characteristics, it is also desirable to store additional printer calibration functions. This allows the print software to simulate the behavior of different printers on a single print engine. For example, such transfer functions allow the print software to match an output that would be produced by both an APPLE.RTM. LASERWRITER.TM. and XEROX.RTM. DOCUTECH.TM. on an IBM.RTM. InfoPrint.TM. printer. Multiple transfer functions can also be used to control other types of behavior such as the compression and expansion of dynamic range.
Although the transfer function for a given printer can be determined a priori, it is subject to variation over time, humidity, temperature, and other factors. Hence, high quality printing often requires that the calibration curves be updated on a regular basis.
Unfortunately, in a network environment, periodically recalibrating all of the printer transfer functions used in a network configuration can be a difficult and time-consuming process. For example, some of the subject printers may be in a secured or remote location. Therefore, there is a need for a system which provides for a convenient propagation of updated printer calibration across all supported printer personalities. The present system satisfies that need.